The present invention relates to an RF-ID receiver system which is compatible with both surface acoustic wave and semiconductor memory based RF-ID tags, thereby allowing multiple RF-ID tag type environments to exist.
A number of different schemes are known for encoding, transmitting and decoding identification signals from RF-ID tags. However, these schemes are generally incompatible, therefore requiring proprietary readers to accept encoded transmissions from tags of the same vendor. Even where the transmission scheme is not proprietary, there is no standardization in the various RF-ID applications.
These RF-ID tags comprise, at a minimum, an antenna and a signal transforming device, some known devices are very complex. There are two particular types of passive RF-ID tags which are used. A first type includes an electronic circuit, e.g., CMOS, to store digital ID data which is then modulated onto a received signal by means of an RF circuit, e.g., a GaAs MESFET, transistor or controlled diode. Power for the data storage and modulating circuit may be derived from an interrogating RF beam or another power source, and power for the transmission itself is also derived from the beam. In this type of system, the interrogating RF beam is generally of fixed frequency, with the resuting modulated signal at the same or a different frequency, with ALM, FM, PSK, QAM or another known modulation scheme employed. In order to provide separation between the received and transmitted signals, the modulated output may be, for example, a harmonic of the interrogating RF beam. Such a system is disclosed in U.S. Pat. No. 4,739,328, expressly incorporated herein by reference.
A known RF-ID interrogation system provides an interrogation signal which incorporates phase diversity, i.e., a phase which periodically switches between 0xc2x0 and 90xc2x0 so that a null condition is not maintained for a period which would prevent RF-ID tag readout with a homodyne receiver. See, U.S. Pat. No. 3,984,835, incorporated herein by reference.
Likewise, a known system, described in U.S. Pat. No. 4,888,59 1, incorporated herein by reference, provides a semiconductor memory tag which is interrogated with a direct sequence spread spectrum signal, which allows discrimination of received signals based on signal return delay. By employing a direct sequence spread spectrum having a decreasing correlation of a return signal with the interrogation signal as delay increases, more distant signals may be selectively filtered. This system employs a homodyne detection technique with a dual balanced mixer.
A second type of RF-ID tag includes a surface acoustic wave device, in which an identification code is provided as a characteristic time-domain reflection pattern in a retransmitted signal, in a system which generally requires that the signal emitted from an exciting antenna be non-stationary with respect to a signal received from the tag. This ensures that the reflected signal pattern is distinguished from the emitted signal. In such a device, received RF energy, possibly with harmonic conversion, is emitted onto a piezoelectric substrate as an acoustic wave with a first interdigital electrode system, from whence it travels through the substrate, interacting with reflector elements in the path of the wave, and a portion of the acoustic wave is ultimately received by the interdigital electrode system and retransmitted. These devices do not require a semiconductor memory. The propagation velocity of an acoustic wave in a surface acoustic wave device is slow as compared to the free space propagation velocity of a radio wave. Thus, assuming that the time for transmission between the radio frequency interrogation system is short as compared to the acoustic delay, the interrogation frequency should change such that a return signal having a minimum delay may be distinguished, and the interrogation frequency should not return to that frequency for a period longer than the maximum acoustic delay period. Generally, such systems are interrogated with a pulse transmitter or chirp frequency system.
Systems for interrogating a passive transponder employing acoustic wave devices, carrying amplitude and/or phase-encoded information are disclosed in, for example. U.S. Pat. Nos. 4,059,831; 4,484,160; 4,604,623; 4,605,929; 4,620,191; 4,623,890; 4,625,207; 4,625,208; 4,703,327; 4,724,443; 4,725,841; 4,734,698; 4,737,789; 4,737,790; 4,951,057; 5,095,240; and 5,182,570, expressly incorporated herein by reference. The tags interact with an interrogator/receiver apparatus which transmits a first signal to, and receives a second signal from the remote transponder, generally as a radio wave signal. The transponder thus modifies the interrogation signal and emits encoded information which is received by the interrogator/receiver apparatus.
Because the encoded information normally includes an identification code which is unique or quasi-unique to each transponder, and because the transponders of this type are relatively light weight and small and may be easily attached to other objects to be identified, the transponders are sometimes referred to as xe2x80x9clabelsxe2x80x9d or xe2x80x9ctagsxe2x80x9d. The entire system, including the interrogator/receiver apparatus and one or more transponders, which may be active or passive, is therefore often referred to as a xe2x80x9cpassive interrogator label systemxe2x80x9d or xe2x80x9cPILSxe2x80x9d.
Other passive interrogator label systems are disclosed in the U.S. Pat. Nos. 3,273,146; 3,706,094; 3,755,803; and 4,058,217.
In its simplest form, the systems disclosed in these patents include a radio frequency transmitter capable of transmitting RF pulses of electromagnetic energy. These pulses are received at the antenna of a passive transponder and applied to a piezoelectric xe2x80x9claunchxe2x80x9d transducer adapted to convert the electrical energy received from the antenna into acoustic wave energy in the piezoelectric material. Upon receipt of a pulse, an acoustic wave is generated within the piezoelectric material and transmitted along a defined acoustic path. This acoustic wave may be modified along its path, such as by reflection, attenuation, variable delay, and interaction with other transducers.
When an acoustic wave pulse is reconverted into an electrical signal it is supplied to an antenna on the transponder and transmitted as RF electromagnetic energy. This energy is received at a receiver and decoder, preferably at the same location as the interrogating transmitter, and the information contained in this response to an interrogation is decoded. The tag typically has but a single antenna, used for both receiving the interrogation pulse and emitting an information bearing signal.
In general, a passive interrogator label system includes an xe2x80x9cinterrogatorxe2x80x9d for transmitting a first radio frequency signal; at least one transponder which receives this first signal, processes it and sends back a second radio frequency signal containing encoded information; and a receiver, normally positioned proximate to or integrated with the interrogator, for receiving the second signal and decoding the transponder-encoded information.
Known technologies allow separate interrogation systems to operate in close proximity, for example by employing directional antennas and employing encoded transmissions, such as a direct sequence spread spectrum signal, which has reduced self-correlation as relative delay increases, thus differentiating more distant signals.
In known passive transponder systems, the encoded information is retrieved by a single interrogation cycle, representing the state of the tag, or obtained as an inherent temporal signature of an emitted signal due to internal time delays.
In the acoustic wave tags described above, the interrogator transmits a first signal having a first frequency that successively assumes a plurality of frequency values within a prescribed frequency range. This first frequency may, for example, be in the range of 905-925 MHz, referred to herein as the nominal 915 MHz band, a frequency band that may be available. The response of the tag to excitation at any given frequency is distinguishable from the response at other frequencies. Further, because the frequency changes over time, the received response of the tag, delayed due to the internal structures, may be at a different frequency than the simultaneously emitted signal, thus reducing interference.
Passive transponder encoding schemes include control over interrogation signal transfer function H(s), including the delay functions f(z). These functions therefore typically generate a return signal in the same band as the interrogation signal. Since the return signal is mixed with the interrogation signal, the difference between the two will generally define the information signal, along with possible interference and noise. By controlling the rate of change of the interrogation signal frequency with respect to a maximum round trip propagation delay, including internal delay, as well as possible Doppler shift, the maximum bandwidth of the demodulated signal may be controlled.
The following references are hereby expressly incorporated by reference for their disclosure of RF modulation techniques, transponder systems, information encoding schemes, transponder antenna and transceiver systems, excitation/interrogation systems, and applications of such systems: U.S. Pat. Nos. 2,193,102; 2,602,160; 2,774,060; 2,943,189; 2,986,631; 3,025,516; 3,090,042; 3,206,746; 3,270,338; 3,283,260; 3,379,992; 3,412,334; 3,480,951; 3,480,952; 3,500,399; 3,518,415; 3,566,315; 3,602,881; 3,631,484; 3,632,876; 3,699,479; 3,713,148; 3,718,899; 3,728,632; 3,754,250; 3,798,641; 3,798,642; 3,801,911; 3,839,717; 3,859,624; 3,878,528; 3,887,925; 3,914,762; 3,927,389; 3,938,146; 3,944,928; 3,964,024; 3,980,960; 3,984,835; 4,001,834; 4,019,181; 4,038,653; 4,042,906; 4,067,016; 4,068,211; 4,068,232; 4,069,472; 4,075,632; 4,086,504; 4,114,151; 4,123,754; 4,135,191; 4,169,264; 4,197,502; 4,207,518; 4,209,785; 4,218,680; 4,242,661; 4,287,596; 4,298,878; 4,303,904; 4,313,118; 4,322,686; 4,328,495; 4,333,078; 4,338,587; 4,345,253; 4,358,765; 4,360,810; 4,364,043; 4,370,653; 4,370,653; 4,388,524; 4,390,880; 4,471,216; 4,472,717; 4,473,851; 4,498,085; 4,546,241; 4,549,075; 4,550,444; 4,551,725; 4,555,618; 4,573,056; 4,599,736; 4,604,622; 4,605,012; 4,617,677; 4,627,075; 4,641,374; 4,647,849; 4,654,512; 4,658,263; 4,739,328; 4,740,792; 4,759,063; 4,782,345; 4,786,907; 4,791,283; 4,795,898; 4,798,322; 4,799,059; 4,816,839; 4,835,377; 4,849,615; 4,853,705; 4,864,158; 4,870,419; 4,870,604; 4,877,501; 4,888,591; 4,912,471; 4,926,480; 4,937,581; 4,951,049; 4,955,038; 4,999,636; 5,030,807; 5,055,659; 5,086,389; 5,109,152; 5,131,039; 5,144,553; 5,163,098; 5,193,114; 5,193,210; 5,310,999; 5,479,160; and 5,485,520. In addition, foreign patents CH346388; DE1295424; DE2926836; DE969289; EP0207020; FR2260115; GB1130050; GB1168509; GB1187130; GB2103408; GB2247096; GB774797; GB987868; JP0138447; JP0189467; JP116054; JP5927278; and NE1566716, as well as the following references: xe2x80x9cIBM,Technical Disclosure Bulletinxe2x80x9d, (vol. 20, No. 7; 12/77), pp. 2525-2526; xe2x80x9cEEE Transactions on Vehicular Technologyxe2x80x9d, (vol. VT-26, No. 1), 2/77; p. 35; A. R Koelle et al. xe2x80x9cShort-Range Radio-Telemetry for Electronic Identification using Modulated RF Backscatterxe2x80x9d, by A. (Proc. of IEEE, 8/75; pp. 1260-126 1).; Baldwin et al., xe2x80x9cElectronic Animal . . . Monitoringxe2x80x9d, 1973; Electronic Letters, December 1975, vol. 11, pp. 642-643; Encyclopedia of Science and Technology; vol. 8, pp. 644-647 (1982); Federal Information Processing Standards Publication 4A, Jan. 15, 1977, Specifications for the Data Encryption Standard; IEEE Transactions, Henoch et al., vol. MTTT-19, No. 1 January 1971; IEEE Transactions, Jaffe et al., pp. 371-378, May 1965; IRE Transactions, Harrington, pp. 165-174, May 1962; IRE Transactions, Rutz, pp. 158-161, March 1961; J. Lenk, Handbook of Microprocessors, Microcomputers and Minicomputers; p. 51 (1979); Koelle et al., xe2x80x9cElectronic Identification . . . Monitoringxe2x80x9d, 7/73 to 6/74, pp. 1-5; P. Lorrain et al., EM Fields and Waves; Appendix A, (1970); Proceedings of IRE, March 1961, pp. 634-635; R. Graf, Dictionary of Electronics; p. 386, (1974); RCA Review, vol. 34, 12/73, Klensch et al., pp. 566-579; RCA Review, Sterzer, 6/74, vol. 35, pp. 167-175; Reports on Research, September-October 1977, vol. 5, No. 2 each of which is expressly incorporated herein by reference.
The present invention provides a system providing a non-stationary radio frequency emission and a receiver system capable of resolving both delay modulation tags, e.g., surface acoustic wave tags, and state machine tags, e.g., semiconductor-based memory tags. The receiver must therefore determine a type of tag, if any, within an interrogation window, and subsequently track a reradiated signal which is received simultaneously, and which is modulated both based on the emitted non-stationary frequency sequence and the internal modulation scheme, as well as a reradiated signal which may be delayed in time.
Since the use of non-stationary radio frequency interrogation signals and subsequent analysis of time domain delay modulated return signal components is conventional, these known methods will not be explored herein in detail. It is understood, however, that the present technique may be used to combine various different RF-ID techniques either in a single hybrid tag system or in an environment with differing types of tags.
In addition, the present invention allows the use of spread spectrum technology to receive data from backscatter tags. Further, certain interactive tags which download information from the interrogation signal may also be compatible with the technique. In fact, since the non-stationary sequence of the interrogation signal is normally ignored by the tag, the sequence itself may be modulated to provide an information signal. The use of a non-stationary frequency for backscatter tags is not heretofore known.
Typically, a state machine passive backscatter RF-ID tag provides an antenna which interacts with a received radio frequency signal. A modulator alters the reflection or impedance characteristic of the antenna system, such that a backscatter signal which varies over time is emitted. The return signal is thus monitored for an information transmission protocol and a message extracted. Since these systems typically are open loop, i.e., no feedback that a message has, in fact, been received by the receiver, redundant or continuous transmissions are made. In order to increase the signal to noise ratio, the return signal is typically modulated using other than simple AM modulation. Where the excitation signal is non-stationary, or the tag distant or moving with respect to the transmitter or receiver, phase locking the receiver to the transmitter may be ineffective as a demodulation scheme. Therefore, the present invention provides a system which tracks the modulation signal of the backscatter signal while effectively ignoring signal components, such as non-stationary frequency, movement induced Doppler effects, and the like, which occur outside the symbol transition rate range of the tags.
Advantageously, in one embodiment, the receiver for a PSK modulated tag need not operate in phase synchronous manner with the radio frequency carrier. At the receiver, the signal from the tag is mixed with a signal which corresponds to the interrogation signal. A dual balanced mixer generates outputs corresponding to both I and Q, however, the strongest phase is analyzed, and the weaker phase is ignored or analyzed, to the extent that it is expected to contain useful information. Because of many variables, the stronger phase may change many times during receipt of a message. In the case of more complex modulation schemes, it may be necessary to analyze the return signal more rigorously. However, this may be accomplished using known signal analysis techniques.
Various advantages of spread spectrum communications may be obtained according to the present invention. First, a receive may be sensitive to the presence of interfering signals in the environment, especially frequency stationary sources, and avoid employing these frequencies, or ensuring that each tag is interrogated for each portion of a complete cycle outside an interference scope. Further, by providing a common band broader than necessary for any one transponder system, a number of interrogation systems may share the same band and environment with low risk of interference. So long as the interrogation sequences are non-overlapping, or uncorrelated, operation will be generally reliable, without need for coupling the interrogation systems. Of course, the interrogation systems may also be coupled, to ensure that there is little or no interference.
In a preferred embodiment of the invention, an excitation transmitted waveform for detecting the reradiated radio frequency signal is a chirp, i.e., a signal which repetitively monotonically changes in frequency over a range. For example, a sawtooth signal may provide an input to a voltage controlled oscillator. In this case, the phase of the chirp signal is continuous, with a change in relative phase angle over time until a limit is reached. In these systems, it is expected that the range of change in frequency is significant and the rate of frequency change is high. Therefore, approximations which rely on a slowly varying signal or small range of variation are inapplicable.
The chirp signal, derived from the excitation signal source, is mixed with a local oscillator signal for downconversion, generating I and Q intermediate frequency (IF) signals. The IF signals are, in turn, detected with an AM detector. In this case, the phase of the IF signal is not stable with respect to the local oscillator, and thus the signal power will migrate between the I and Q phases. Therefore, the preferred embodiment analyzes both the detected I and Q signal, to determine the data encoded on the received waveform. For example, the stronger signal may be presumed to have the signal with the highest signal to noise ratio, and therefore used exclusively in the signal analysis. The I and Q signals may also be analyzed together. Since the phase is presumed to be instable, and in fact may be rotating, the stronger of the I and Q signals will oscillate.
This method may also have applicability to other types of modulation schemes which do not employ quadrature phase modulation techniques, e.g., QAM, such that any one phase of the demodulated signal includes all of the modulated information of interest.
By allowing a modulated backscatter radio-frequency identification tags to coexist in an environment with surface acoustic wave identification tags, the present invention simplifies system operation with differing tag types and allows a system to be established with a future change in preferred tag type, without redundant or incompatible equipment.
In order to read a known type of SAW RF-ID tag, e.g., an acoustic transponder available from XCI, Inc, San Jose, Calif., a non-stationary frequency radio frequency interrogation signal is transmitted to the tag, where it is modified by the SAW device, such as by reflecting and/or delaying portions of the wave so that a return signal is modulated. In environments including multiple tag types, the type of tag is typically unknown until a response is received. Therefore, it is an object according to the present invention to accept and decode a return signal from a semiconductor memory RF-ID tag from irradiation with a frequency modulated radio frequency interrogation beam, while also accepting and decoding a response from a surface acoustic wave RF-ID tag, and determining a tag type and encoding, or an absence of a valid tag. For example, the absence of tag detector includes an event detector, e.g., a car in a toll lane, in conjunction with no output from decoders for the different types of transponder supported. Optionally, the system may determine the validity of a code, so that an invalid code may be distinguished from an absent code, with possible different processing.
Of particular note in the present invention, the xe2x80x9ccarrierxe2x80x9d frequency is not stationary, and therefore the receiver is capable of receiving digitally modulated backscatter signals which are immediately modulated and retransmitted, without substantial delay, as well as reradiated radio frequency signals in which the encoding is presented as one or more substantial delays of a retransmitted derivative of the excitation signal, e.g., from a SAW-based RF tag. In the latter case, by retuning the excitation signal periodically or continuously, the delayed signals may be detected, which would be difficult or impossible if the excitation signal remained at the same frequency. On the other hand, this non-stationary excitation signal requires compensation before a digitally modulated backscatter signal may be detected. Thus, the present invention provides a multiprotocol tag reader, allowing different types of tags to be reliably identified.
In acoustic RF-ID transponder systems, the information code associated with and which identifies the passive transponder is built into the transponder at the time that a layer of metallization is fully defined on the substrate of piezoelectric material. This metallization thus advantageously defines the antenna coupling, launch transducers, acoustic pathways and information code elements, e.g, reflectors. Thus, the information code in this case is non-volatile and permanent. The information is present in the return signal as a set of characteristic perturbations of the interrogation signal, such as delay pattern and attenuation. In the case of a tag having launch transducers and a variable pattern of reflective elements, the number of possible codes is Nxc3x972M where N is the number of acoustic waves launched by the transducers and M is the number of reflective element positions for each transducer. Thus, with four launch transducers each emitting two acoustic waves, and a potential set of eight variable reflective elements in each acoustic path, the number of differently coded transducers is 2048. Therefore, for a large number of potential codes, it is necessary to provide a large number of launch transducers and/or a large number of reflective elements. However, power efficiency is lost with increasing device complexity, and a large number of distinct acoustic waves reduces the signal strength of the signal encoding the information in each. The transponder tag thus includes a multiplicity of xe2x80x9csignal conditioning elementsxe2x80x9d, i.e., delay elements, reflectors, and/or amplitude modulators, are coupled to receive the first signal from a transponder antenna. Each signal conditioning element provides an intermediate signal having a known delay and a known amplitude modification to the first signal. Where the signal is split into multiple portions, it is advantageous to reradiate the signal through a single antenna Therefore, a single xe2x80x9csignal combining element.xe2x80x9d coupled to all of the signal conditioning elements and/or signal portions is provided for combining the intermediate signals to produce the second signal. This second signal is coupled out to the same or a separate antenna for transmission as a reply. As described above, the signal delay elements and/or the signal combining element impart a known, unique informational code to the second signal.
Preferably, the passive acoustic wave transponder tag includes at least one known (control) element, which assists in synchronizing the receiver and allows for temperature compensation of the system. As the temperature rises, the piezoelectric substrate may expand and contract, altering the characteristic delays and other parameters of the tag. Although propagation distances are small, an increase in temperature of only 20xc2x0 C. can produce an increase in propagation time by the period of one entire cycle at a transponder frequency of about 915 MHz. The acoustic wave is often a surface acoustic wave, although bulk acoustic wave devices may also be constructed.
The receiving and decoding apparatus associated with the system includes apparatus for receiving the second signal from the transponder and a mixer arranged to receive both the first signal and the second signal for performing, four quadrant multiplication of these two signals. The mixer is preferably a complex mixer, generating I and Q phases 90xc2x0 apart, although the mixer may be polyphasic (having two, three or more phases) which may be symmetric or asymmetric. The difference frequencies (or frequencies derived from the difference frequencies) of the first and second signals, respectively are then processed by one or more signal processors, to detect the phases and amplitudes of the respective difference frequencies contained in the third signal in the case of an acoustic transponder, or a sequence of modulation states in the case of a semiconductor modulator backscatter transponder, to determine the informational code associated with the interrogated transponder. Where the code is provided as a set of time delays, the signal processor performs a time-to-frequency transform (Fourier transform) on the received signal, to assist in determination of the various delay parameters. The characteristic delays (and phase shifts) of the transducer then appear in the transformed data set at the receiver as signal energy having a time delay. Alternately, a set of matched filters may be implemented, and the outputs analyzed. Where the code is provided as a sequence of symbols, a time domain analysis will generally suffice. The preferred embodiment of the invention employs separate analyzer circuitry for differing encoding schemes, but the circuitry and analysis may also be consolidated into a single system, for example a digital signal processing scheme.
In practice, a passive interrogator label system is frequently configured such that a plurality of transponders are interrogated from a number of locations. For example, if the transponders (labels) are carried on persons who are authorized entry into a building, the transmitting and receiving antennas are normally located near several doors to the building. According to the present invention, the signal analysis of both acoustic and semiconductor based transponders may be remote from the interrogation antenna system
As another example, the labels may be placed on cattle which are monitored at a number of locations, such as a holding area, a feeding area and the like. The labels may also be placed on railroad cars to permit car identification at various locations throughout a switchyard or rail network. Other uses of such systems are known, and in fact the widespread acceptance of interrogation systems, be they passive or active, have generated the problem addressed by the present invention, namely, the presence of many competing and incompatible standards.
Thus, the processing of the transponder signal may be divided between the interrogator-transponder communication in the radio frequency range, and the decoding of the received information, with the two functions potentially separated. The decoding system electronics may be multiplexed to effectively service a number of locations efficiently through a network.
It is also an object of the invention to provide a versatile receiver which can extract additional information from a return signal and selectively communicate with a plurality of RF-ID tags simultaneously, e.g., a first semiconductor memory tag and a second SAW reflector pattern memory tag.
It is a further object according to the present invention to provide a tag which includes both semiconductor memory and electrode pattern memory.
It is also an object of the invention to provide a method for interrogating a backscatter generating tag, comprising the steps of (a) generating an interrogation signal having a frequency within an interrogation band; (b) emitting the interrogation signal as a radio wave signal; (c) interacting the emitted radio wave signal with a backscatter generating tag; (d) receiving a radio frequency backscatter signal from the tag; (e) mixing the received backscatter signal with a plurality of representations of the interrogation signal, each of said plurality of representations differing in phase, to produce a plurality of mixed signals; (f) comparing a respective signal strength of said plurality of mixed signals; and (h) analyzing said difference signals over time to determine a significant information sequence of the backscatter signal, while discounting an importance of at least one of the plurality of mixed signals at any given time based on said compared respective signal strengths.
It is a further object of the invention to provide a dual mode tag identification system, in which a reradiated representation of an interrogation signal is analyzed in a first mode to determine a transfer function for said interrogation signal and in a second mode to determine a time sequence of modulation states imposed on said interrogation signal.
It is still another object of the invention to provide a transponder interrogation system for interrogating a transponder which receives a radio frequency wave and emits a modified radio frequency wave, comprising an interrogation radio frequency wave generator, generating a radio frequency excitation pulse adapted for probing a plurality of characteristic time-constants of a transponder and for communicating with the transponder; an antenna, for receiving the modified radio frequency wave; a first decoder, for determining the plurality of characteristic time-constants from the modified radio frequency wave; and a second decoder for determining a sequence of modulation states from the modified radio frequency wave.
It is also an object of the present invention to provide a backscatter transponder interrogation system, comprising: (a) an input for receiving a backscatter signal from a backscatter transponder due to an interrogation signal; (b) a multiphase mixer for mixing said received backscatter signal and a representation of said interrogation signal to produce multiphasic outputs; (c) means for selecting a mixer multiphasic output having substantial signal strength; and (d) a decoder for decoding a sequence of symbols from the selected mixer multiphasic output.
It is another object of the present invention to provide an RF-ID tag interrogator, responsive to a return signal from an RF-ID tag having a semiconductor device outputting symbols which are accessed serially over time to sequentially modulate an interrogation signal at a modulation rate, comprising: a transmitter, transmitting a radio frequency interrogation signal, said interrogation signal having a frequency which substantially varies over time; a receiver, receiving a signal from the RF-ID tag which corresponds to said radio frequency interrogation signal, sequentially modulated over time based on the symbols; a decoder, having: a phase-sensitive demodulator, for extracting a complex modulation pattern from said received signal, with respect to a representation of said interrogation signal; a symbol detector receiving said complex modulation pattern, extracting a data clock from one of said complex modulation pattern, said interrogation signal, or a reference clock, compensating for a phase rotation in the complex modulation pattern due to frequency variation of said interrogation signal at a rate faster than the modulation rate, and extracting said symbols from said compensated complex modulation pattern and said data clock.
The present invention also provides as an object a radio frequency receiving device, operating in an environment including an RF generator, generating a time-variant RF signal which propagates through space, and an RF signal modulator having a frequency modulation pattern based on data symbols stored in said device, comprising: an input, receiving a frequency modulated signal corresponding to said time-variant RF signal modulated by the data symbols; a demodulator, producing a demodulated signal by mixing a signal corresponding to said time-variant RF signal with said received signal, while preserving a phase pattern; a comparator, selecting a phase component having a greatest magnitude from at least two phase components having differing phase axes of said demodulated signal, said comparator having a magnitude selectivity pattern excluding selection of a component based primarily on a pattern of said data symbols; a detector, detecting said selected phase component to extract said data symbols; and an output, for outputting information relating to said data symbols.
It is a further object of the invention to provide a device for receiving information from a remote tag, the tag having information stored in a memory and a modulator for frequency modulating an incident signal based on the stored information, comprising: a transmitter for transmitting a radio frequency carrier having a time varying center frequency in proximity to the tag; a receiver for receiving a frequency modulated, time varying center frequency carrier signal from the tag; a balanced mixer, receiving said frequency modulated, time varying center frequency carrier signal and said radio frequency carrier to produce at least a difference signal with at least two outputs each representing a different phase axis; a detector circuit receiving said at least two outputs and extracting the information from at least one of said outputs.
It is a still further object of the invention to provide a radio frequency receiving device, operating in an environment including an RF generator, generating a phase-continuous, time-variant RF signal which propagates through space, and an RF signal modulator having a frequency modulation based on data symbols, comprising: an input, receiving a frequency modulated signal corresponding to said time-variant RF signal modulated by the data symbols; a demodulator, producing a demodulated signal by mixing a signal corresponding to said time-variant RF signal with said received signal, while preserving a phase pattern; a comparator, selecting a phase component having a greatest magnitude from at least two phase components having differing phase axes of said demodulated signal, said comparator having a magnitude selectivity pattern excluding selection of a component based primarily on a pattern of said data symbols; a detector, detecting said selected phase component to extract said data symbols; and an output, for outputting information relating to said data symbols.
It is also an object of the present invention to provide an RF-ID tag reader, responsive a return signal from an RF-ID tag having an RF output modulating an interrogation signal over time in a pattern corresponding to a sequence of symbols, comprising: a receiver, receiving a modulated signal from the RF-ID tag which corresponds to said radio frequency interrogation signal, modulated over time based on the symbols; a complex demodulator, for demodulating in complex space a modulated signal pattern of the received modulated signal to produce at least two phases and preferentially producing an output based on a phase having a greater signal strength, to extract a modulation pattern from said demodulated signal; an analyzer for reconstructing the symbols from the detected modulation pattern; an output for producing information corresponding to said sequence of symbols.
It is stiff further object of the invention to provide a RF-ID tag backscatter demodulator having a signal relative phase change detector for determining a relative phase change in a received signal. Preferably, quadrature phase representations of the signal are compared with respective delayed quadrature representations to detect a relative phase reversal edge, with analysis of the quadrature phase edge signals based on a quadrature phase received signal strength.
These and other objects will become apparent from a review of the detailed description of the preferred embodiments.